The near impossibility of getting very high quality book proposals published. Even very accomplished photographers, Magnum members (as a nod to the OP), others with international reputations have a very hard time getting books published.
During the 60s-80s publishers would take on interesting or compelling photo book projects because they knew they could make money and produce a nice book with an edition of 2,000 or 3,000. People would buy the books, production values would be pretty high, etc.
Today print runs need to be much higher, far fewer photo books are being produced by far fewer publishers willing to make the investment. No one is satisfied with a modest profit, publishers want a huge profit. Costs of paper, ink, and printing have skyrocketed.
As a result of all this, the books themselves tend to be much more generic or have a broader appeal, such as books on celebrities or pretty Irish landscapes, lingerie, or genitals (take a look at Taschen's "Sexy Books") etc.
And book buyers have become more fussy. A black and white photo book printed in halftone would be dead in the water. Buyers won't even consider a b&w photo book unless it is at least duotone.
Today, even world-class artist/photographers need to approach potential publishers with grant money or other funding.
I could go on and on. There are, of course, exceptions to everything. But there's not doubt this is the trend.